Hervé Eon, 61, a leftwing activist, held up his sign as Sarkozy's motorcade drove past during a presidential visit to Laval, western France, in 2008. The small A4-sized cardboard sign did not feature Sarkozy's name but said simply: "Casse-toi pov'con."

The line, which loosely means "get lost, you prat" had been uttered by Sarkozy months earlier when a man refused to shake his hand at an agricultural fair, causing media outrage at his non-presidential language and demeanour. It later became a widely used political slogan against the president used by the left on stickers and posters.

Eon, a former social worker, was arrested as soon as he got out his sign.

The French state prosecutor brought a case against him for offence against a head of state, and he was ordered to pay a symbolic fine of €30 and given a criminal conviction. But European human rights court judges found the sign was of a satirical nature and ruled it did not warrant a criminal conviction.

Some French senators warned recently that scrapping the offence of offending the head of state would leave a legal void. So parliamentarians agreed that the president would from now on fall into the same category as ministers and MPs: against whom there is an offence of slander or defamation.

This crime is punishable by a fine of up to €45,000 (£39,000). The state prosecutor could open a case but only at the demand of the president.

The offence of insulting the French head of state was invoked several times under General de Gaulle, but was largely forgotten under the presidents who followed. François Mitterrand and Jacques Chirac never used it. Sarkozy was the first to bring a case in decades.

Until 2000, the offence carried a possible prison sentence of up to a year.

At the time of Eon's conviction, the then hard-left senator Jean-Luc Melenchon campaigned to have the offence scrapped, warning it was a republican throwback to the old offence of "insulting the monarch" under the ancien regimé.